It's more the engine than the platform that I don't like. They've crammed a lot of "this should work" things onto it and then put in a wet belt for the oil pump and sandwich it and the high pressure fuel pump between the transmission. Good thinking, lets pull the tranny to replace a belt. Most people know diesels love to run for extended periods of time and love to run hot so adding auto stop/start to this engine seems counterintuitive. And IMO they sound like ****. They sound like a badly misfiring gasser, or a 5.3 with lifter problems
The real duramax 6.6 engines on the other hand are awesome. Other than the early fuel injector problems these things are beast mode. I know lots of people that have them and love them